I've done lots of trailers with plain old Rustoleum over Red Metal Primer and they have lasted me a very long time. I store them outside without a cover and it still seems to hold up. As said, prep is everything no matter what paint you use. If you get a good prep you will get a good finish. I usually use a good brush so I can get in all the tight spots where a sprayer tends to leave bare... if it is thinned properly it will flow and lay down pretty flat but 30% might be bit much thinning if you are going to brush it on.
And don't discount spraying down the area with a spray nozzle on a hose just before painting to keep the dust down. I've painted vehicles outside and the paint came out great without a tent or any other protection- just a good use of a garden hose to keep the dust down before painting and hopefully a fairly still day...
